The invention relates to overvoltage protection for a thyristor and, more particularly, to an improved overvoltage protection circuit including an avalanche diode for diverting a specified amount of overvoltage energy from a thyristor and a cooperating simulator circuit and ignition pulse device adapted to ignite the thyristor to divert overvoltage energy and discharge the avalanche diode when the load on the diode exceeds a specified safe operational amount.
Thyristors are used in electrical valves, for example for high-voltage convertors, wherein many thyristors may be connected in series and overvoltages may occur across all of the series connected thyristors or across individual thyristors in the chain, due to an uneven voltage distribution over the thyristors. Such overvoltages may permanently damage the thyristors by inducing self-ignition.
It is known in the art to divert overvoltage energy from a thyristor to prevent the damaging overvoltage-induced self-ignition by using a diverter circuit connected in parallel with the thyristor to divert the entire overvoltage energy away from the thyristor. However, the components of such diverters must necessarily be dimensioned to operate under high energy conditions.
Since a properly ignited thyristor is usually able to pass the overvoltage energy without damage, it has been suggested that high energy diverters may be avoided by using a diverter circuit with components of relatively small dimensions to detect the overvoltage condition and to thereafter activate an ignition pulse device to ignite the thyristor to pass the overvoltage energy.
However, a disadvantage of this overvoltage protection scheme is that the thyristor must be quickly ignited after the initial sensing of an overvoltage condition and therefore, the thyristor is often needlessly ignited in response to low energy overvoltages and unnecessary disturbances are thereby induced in the thyristor circuit.
Accordingly, it is an object of the invention to provide a simple and effective means to protect a thyristor from overvoltage-induced self-ignition without needlessly igniting the thyristor in response to low energy overvoltages.
A further object of the invention is to provide a thyristor overvoltage protection means wherein the discharge capacity of the diverter is fully utilized.
Another object of the invention is to provide a thyristor overvoltage protection means that compensates for the non-ideal electrical characteristics of an avalanche diode diverter.
A further object of the invention is to provide an overvoltage protection means that will operate to protect reversible thyristor valves against overvoltages.
These and other objects of the invention will become apparent from a review of the detailed specification which follows and a consideration of the accompanying drawings.